The subscriber capacity of a mobile radiotelephone system is limited by the frequency spectrum allocated to such use. To optimize the subscriber capacity of a given system it is critical that the allocated frequency spectrum be used with the highest possible efficiency. Improvements in this efficiency may be attained by application of various modulation and coding techniques in the transmission system to achieve the highest efficient use of the available frequency spectrum.
Increasing the use of the available frequency spectrum has been greatly enhanced by application of the cellular concept to radiotelephone systems. With the cellular concept, the area served by a radiotelephone provider is divided into contiguous geographical cells each being served by a particular set of channels. Cells separated from one another by a certain number of intervening cells, with a distance large enough to prevent transmission interference, normally can reuse the same transmission channels. Such channel reuse greatly enhances the efficient use of the available frequency spectrum.
To fully realize the advantages of the cellular concept requires that the various transmission frequencies or channels be effectively allocated to the various cells in a manner which utilizes the available spectrum as efficiently as possible. Such frequency/channel allocations have traditionally been assigned to the various cells by using the channel set concept in which each cell is assigned a particular set of channels.
In the traditional methods of allocation, cell coverage is assumed to be a regular polygon and traffic usage is assumed to be uniformly distributed. The polygons are all considered to be uniformly tiled to cover a geographic service area. For purposes of initial channel assignment the actual geographic terrain and actual RF propagation is ignored. Channels are equally assigned in sets of channels to the various cells until all the cells have been assigned the total number of channels. At this point adjustment is made to account for the actual irregular geographic cell layout and the actual terrain irregularities. Further adjustment considers any unequal service demand among the cells and specifically attempts to minimize co-channel and adjacent channel interferences. This method of channel assignment normally involves so many initial gross assumptions that the final result is much below the desired efficiency of spectrum use.
Some non regular channel assignment techniques have attempted to exploit the differences in geographic terrain, RF propagation and service demand to permit a more efficient allocation of the available frequency spectrum. These methods may not partition the spectrum into sets of channels and does not proscribe any particular reuse pattern. These assignment techniques have been mostly ad hoc to date and are normally highly labor intensive in application since they are for the most part applied manually. These techniques do not readily permit adjustment of channel allocations to accommodate changes or growth in service demand.
To permit full utilization of the allotted spectrum requires an allocation system that permits an allocation of channels according to a designed scenario and actual physical conditions and one which readily adapts to changes in service demand.